This invention relates to the recovery of natural gas from coal seams and, more particularly, to a well treatment fluid and method of stimulating gas production from subterranean coal beds by hydraulic fracturing.
Subterranean coal beds often contain large quantities of methane. The presence of methane in these subterranean coal deposits presents a safety hazard in coal mining operations, but also presents an opportunity for recovery of a valuable fuel. In the past, coalbed methane was often vented to the atmosphere or flared to reduce the safety risk in mining. More recently, in order to minimize air pollution and maximize economic return from coal bed operations, there has been an increasing focus on recovering methane rather than venting or flaring it. The recovery of coalbed methane is typically accomplished by drilling and completing a gas well into the coal seam and fracturing the well within the coal formation to enhance methane recovery.
Hydraulic fracturing methods for oil and gas wells drilled in a hard rock formation involve injecting a fracturing fluid (e.g., an aqueous gel or an aqueous foam) through the wellbore and against the face of the subterranean formation at pumping rates and pressures sufficient to create or extend cracks in the formation. Typically a proppant (e.g., sand or bauxite) is mixed with the fracture fluid and is carried by the fluid into the fractures. When the pumping rate and pressure are reduced, the fractured formation settles back onto the emplaced proppant, and the proppant holds the fractures open sufficiently to establish a permeable fluid communication channel from the tip of the pack of proppant back to the wellbore.
Fracture stimulation of coalbed methane reservoirs requires techniques quite different from those used in conventional hard-rock reservoirs. The methane in a coal seam is adsorbed to the surface of the coal. At a certain pressure, governed by the Langmuir desorption isotherm, the methane will begin to desorb from the coal. In addition, coal seams are often completely saturated with water. In these cases, large quantities of water must be removed in order to lower the reservoir pressure to a point below the methane desorption pressure. Therefore, a hydraulic fracturing treatment in a coal seam must be designed to produce water effectively.
Maintaining the coal in an oil-wet state facilitates water production. This is because coal is soft and friable. Wells are generally produced at maximum pressure drawdown to reduce the reservoir pressure as quickly as possible. The proppant particles (usually sand) become embedded into the fracture faces due to the increase in closure stress created by the high drawdown pressure. Proppant embedment causes a large quantity of coal fines to be produced. If these fines are water-wet, then they will be easily transported in the water phase during dewatering of the coal bed. The fines will then migrate into the fracture, eventually causing severe reduction of the fracture conductivity. It is therefore important to maintain the coal fines in an oil-wet state, so they will tend to clump together in the presence of water, thereby greatly reducing their mobility. This concept is also critical in the natural fracture (cleat) system of the coal adjacent to the hydraulic fracture. Coal fines will be generated due to shrinkage of the coal, oxidation, etc. These fines can cause plugging of the cleat system, which severely reduces the well productivity and ultimate gas production.
Additives exist that can provide good oil wetting of coal. For example, superior oil wetting in the presence of water can be achieved by methods and materials described in U.S. Pat. No. 5,229,017 (Nimerick and Hinkel). One such commercially available surfactant, denoted surfactant A herein, (available from Schlumberger), comprises a branched tridecyl alcohol with seven moles ethylene oxide (EO) and two moles butylene oxide (BO).
Foamed fracturing fluids are often preferred over non-foamed fracturing fluids in coal seam reservoirs in order to minimize the damage associated with the natural polymers typically present in the base fluid. Nitrogen is most often used as the gaseous phase in the foam fracturing treatments. However, materials that act as good oil-wetters for coal have been proven ineffective in providing stable aqueous foams. For example, surfactant A acts as an anti-foaming agent.
There is a need for improved fracturing fluids and methods that are suitable for use in coal beds to stimulate production of methane.
The present invention relates to a well treatment fluid composition that comprises a carrier fluid, a viscosifying agent, an amphoteric surfactant, and proppant. This fluid composition is especially well suited for use in fracturing gas wells in coal beds and is preferably used in a foam form, that is further comprising a gas such as nitrogen or air.
Preferably, the surfactant comprises an alkyl-aminocarboxylic acid or carboxylate, that is a zwitterionic compound of formula Rxe2x80x94NH2xe2x80x94(CH2)nxe2x80x94C(O)OX, where R is a saturated or unsaturated alkyl group having from 6-20 carbon atoms, n is from 2-6, and X is hydrogen or a salt forming cation. In various specific embodiments of the invention, n can be from 2-4; and R can be a saturated or unsaturated alkyl group having from 10-16 carbon atoms. More preferably, the surfactant is an alkyl-aminopropionic acid or propionate (n=2). One particular preferred surfactant is coco-aminopropionate, of formula RNH2CH2CH2COOX, where R is dodecyl, tetradecyl or hexadecyl, with a distribution of about dodecyl (C12), 40%, tetradecyl (C14), 50% and hexadecyl (C16), 10% and X is for example sodium.
The viscosifying agent can be, for example, a solvatable, crosslinkable polymer selected from the group consisting of guar, hydroxypropyl guar, carboxymethyl guar, carboxymethylhydroxypropyl guar, hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, hydroxypropyl cellulose, xanthan, and mixtures thereof.
The can also include a crosslinking agent, a gel breaker for the viscosifying agent, and one or more other additives.
Another aspect of the present invention is a method of hydraulically fracturing a subterranean coal bed. This method comprises the step of injecting a well treatment fluid composition via a wellbore into a subterranean coal bed at a flow rate and pressure sufficient to produce or extend a fracture in the formation. The well treatment fluid composition can have the components described above. Alternatively, the fluid composition used in the method can be free of the viscosifying agent and/or proppant.
The present invention provides a remedial treatment of coalbed gas wells to enhance dewatering and the production of gas. The invention is useful both for fracturing newly drilled wells and for workover of existing wells (e.g., remedial fracturing of a well that has been producing for some time and has already been fractured in the past).
The surfactants used in the present invention have good oil wetting characteristics in the presence of coal, and are effective foaming agents. Thus, these surfactants are capable of creating a stable, foamed fluid, using either freshwater or brine, while maintaining the natural surface properties of the coal, and can minimize the mobility and migration of coal fines, thereby preserving fracture conductivity and cleat permeability. Additionally, the stability of foams formed with these surfactants should decrease with pH, which will facilitate clean up of the foam after the fracturing treatment (i.e., clean up can be performed with a reservoir fluid having a pH lower than the pH of the foam).